The present invention relates to a comparator circuit for comparing an unknown input voltage with a known input voltage to produce an output signal representing the result of the comparison.
A prior comparator circuit will be described referring to FIG. 1. The emitters of PNP transistors Q1 and Q2 are connected together to a first power source providing a positive potential Vcc, through a constant current source 1. A reference input signal of given reference voltage Vref is applied to the base of the transistor Q1, through the first input terminal 2. An input signal at an unknown voltage Vin is applied to the base of the transistor Q2, through the second input terminal 3. The collectors of the transistors Q1 and Q2 are connected to the collectors of NPN transistors Q3 and Q4, respectively. The transistors Q3 and Q4 cooperate to form a current mirror circuit. The transistors Q3 and Q4 are so arranged that their emitters are grounded, the bases are interconnected, and the collector of the transistor Q3 is connected to the common base of the transistors Q3 and Q4. With such a current mirror arrangement, the emitter current of the transistor Q4 is the same as that of the transistor Q3. Transistors Q1 to Q4 constitute an operational amplifier 4.
Further, an output transistor Q5 of NPN type having a grounded emitter is connected at its base to the collector of the transistor Q4 of the operational amplifier 4. The collector of transistor Q5 is connected to a power source providing a potential Vcc through a resistor 5, and serves as an output terminal 6.
The operation of the illustrated known comparator circuit, assuming application of a reference input signal Vref and an unknown input signal Vin, will be described. When Vin&gt;Vref, the transistor Q1 is conductive while the transistor Q2 is nonconductive. Accordingly, a current I.sub.E1 from the constant current source 1 flows through the emitter-collector path of the transistor Q1 and is amplified to value of .alpha.I.sub.E1. .alpha. is the current amplification factor of the transistor Q1. The current .alpha.I.sub.E1 flows into the transistor Q3, too. The transistors Q3 and Q4 form the current mirror circuit. However, a current of the same value as the current .alpha.I.sub.E1 will now flow into the collector-emitter path of the transistor Q4 through the emitter-collector path of the transistor Q2, because the transistor Q2 is nonconductive. No current is fed from the constant current source 1. The charge stored in the base of the transistor Q5 (the charge stored in the stray capacitance 7 equivalently connected between the base of the transistor Q5 and ground in FIG. 1) discharges through the collector-emitter path of the transistor Q4. As a result, the output transistor Q5 becomes nonconductive since the base-emitter voltage is 0 V. And the output terminal 6 becomes high in potential (the difference between the potential Vcc and the voltage drop across the resistor 5).
When Vin&lt;Vref, the transistor Q1 is nonconductive, while the transistor Q2 is conductive. Accordingly, the current I.sub.E1 from the constant current source 1 flows through the emitter-collector path of the transistor Q2 and is amplified to a value of .alpha.I.sub.E1. Here, .alpha. is the current amplification factor of the transistor Q2. Since the transistor Q1 is nonconductive, no current flows through the transistor Q1 or the transistor Q3. Therefore, the base-emitter voltage of the transistor Q3 is 0 V. Then, the transistor Q4 which cooperates with the transistor Q3 to form the current mirror circuit, becomes nonconductive. The current .alpha.I.sub.E1 amplified by the transistor Q2 flows into the base of the transistor Q5 to render the output transistor Q5 conductive. As a result, the output terminal 6 becomes low in potential (ground level).
As described above, the output transistor Q5 is conductive or nonconductive in accordance with the amplitude of the unknown voltage input signal Vin relative to that of the reference signal Vref so that the comparison result can be obtained by detecting the potential at the collector of the output transistor Q5 at the output terminal 6.
In the construction of the prior comparator circuit, when Vin&lt;Vref and Vin=0 V, erroneous operation can occur for the following reason. The collector-emitter voltage V.sub.CE(Q2) of the transistor Q2 is given by EQU V.sub.CE(Q2) =Vin+V.sub.BE(Q2) -V.sub.BE(Q5)
where V.sub.BE(Q2) and V.sub.BE(Q5) are the base-emitter voltage of the transistors Q2 and Q5, respectively. Since V.sub.BE(Q2) and V.sub.BE(Q5) are substantially equal to each other, when Vin=0 V, V.sub.CE(Q2) is almost 0 V. Under this condition, the transistor Q2 fails to operate normally and instead operates as a diode. The current I.sub.E1 from the constant current source 1 flows into the base of the transistor Q2 but does not flow into the collector. As a result, the transistor Q5 can not maintain its conductive state. Thus, the transistor Q5 operates erroneously.